Re: [PATCH v4 0/5] Write-placement hints and FDP

[Christian Brauner <brauner@kernel.org>]

On Tue, Sep 03, 2024 at 07:58:46PM GMT, Kanchan Joshi wrote:
> Hi Amir,
> 
> 
> On 8/26/2024 10:36 PM, Kanchan Joshi wrote:
> > Current write-hint infrastructure supports 6 temperature-based data life
> > hints.
> > The series extends the infrastructure with a new temperature-agnostic
> > placement-type hint. New fcntl codes F_{SET/GET}_RW_HINT_EX allow to
> > send the hint type/value on file. See patch #3 commit description for
> > the details.
> > 
> > Overall this creates 128 placement hint values [*] that users can pass.
> > Patch #5 adds the ability to map these new hint values to nvme-specific
> > placement-identifiers.
> > Patch #4 restricts SCSI to use only life hint values.
> > Patch #1 and #2 are simple prep patches.
> > 
> > [*] While the user-interface can support more, this limit is due to the
> > in-kernel plumbing consideration of the inode size. Pahole showed 32-bit
> > hole in the inode, but the code had this comment too:
> > 
> > /* 32-bit hole reserved for expanding i_fsnotify_mask */
> > 
> > Not must, but it will be good to know if a byte (or two) can be used
> > here.
> 
> Since having one extra byte will simplify things, I can't help but ask - 
> do you still have the plans to use this space (in entirety) within inode?

I just freed up 8 bytes in struct inode with what's currently in -next.
There will be no using up those 8 bytes unless it's for a good reason 
and something that is very widely useful.